Periodic patterns and technique to control misaligment between two layers

ABSTRACT

A method and system to measure misalignment error between two overlying or interlaced periodic structures are proposed. The overlying or interlaced periodic structures are illuminated by incident radiation, and the diffracted radiation of the incident radiation by the overlying or interlaced periodic structures are detected to provide an output signal. The misalignment between the overlying or interlaced periodic structures may then be determined from the output signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/495,001,filed Jul. 27, 2006; which is a continuation of application Ser. No.11/355,613, filed Feb. 15, 2006, now abandoned; which is a continuationof application Ser. No. 11/062,255, filed Feb. 18, 2005, now abandoned;which is a continuation of application Ser. No. 10/682,544, filed Oct.8, 2003, now abandoned; which is a continuation of application Ser. No.09/833,084, filed Apr. 10, 2001, now abandoned; which applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates in general to metrology systems for measuringperiodic structures such as overlay targets, and, in particular, to ametrology system employing diffracted light for detecting misalignmentof such structures.

Overlay error measurement requires specially designed marks to bestrategically placed at various locations, normally in the scribe linearea between dies, on the wafers for each process. The alignment of thetwo overlay targets from two consecutive processes is measured for anumber of locations on the wafer, and the overlay error map across thewafer is analyzed to provide feedback for the alignment control oflithography steppers.

A key process control parameter in the manufacturing of integratedcircuits is the measurement of overlay target alignment betweensuccessive layers on a semiconductor wafer. If the two overlay targetsare misaligned relative to each other, then the electronic devicesfabricated will malfunction, and the semiconductor wafer will need to bereworked or discarded.

Measurement of overlay misregistration between layers is being performedtoday with optical microscopy in different variations: brightfield,darkfield, confocal, and interference microscopy, as described inLevinson, “Lithography Process Control,” chapter 5, SPIE Press Vol.TT28, 1999. Overlay targets may comprise fine structures on top of thewafer or etched into the surface of the wafer. For example, one overlaytarget may be formed by etching into the wafer, while another adjacentoverlay target may be a resist layer at a higher elevation over thewafer. The target being used for this purpose is called box-in-box wherethe outer box, usually 10 to 30 μm, represents the position of thebottom layer, while the inner box is smaller and represents the locationof the upper layer. An optical microscopic image is grabbed for thistarget and analyzed with image processing techniques. The relativelocation of the two boxes represents what is called the overlaymisregistration, or the overlay. The accuracy of the optical microscopeis limited by the accuracy of the line profiles in the target, byaberrations in the illumination and imaging optics and by the imagesampling in the camera. Such methods are complex and they require fullimaging optics. Vibration isolation is also required.

These techniques suffer from a number of drawbacks. First, the grabbedtarget image is highly sensitive to the optical quality of the system,which is never ideal. The optical quality of the system may produceerrors in the calculation of the overlay misregistration. Second,optical imaging has a fundamental limit on resolution, which affects theaccuracy of the measurement. Third, an optical microscope is arelatively bulky system. It is difficult to integrate an opticalmicroscope into another system, such as the end of the track of alithographic stepper system. It is desirable to develop an improvedsystem to overcome these drawbacks.

SUMMARY OF THE INVENTION

A target for determining misalignment between two layers of a device hastwo periodic structures of lines and spaces on the two different layersof a device. The two periodic structures overlie or are interlaced witheach other. The layers or periodic structures may be at the same ordifferent heights. In one embodiment either the first periodic structureor the second periodic structure has at least two sets of interlacedgrating lines having different periods, line widths or duty cycles. Theinvention also relates to a method of making overlying or interlacedtargets.

An advantage of the target is the use of the same diffraction system andthe same target to measure critical dimension and overlaymisregistration. Another advantage of the measurement of misregistrationof the target is that it is free from optical asymmetries usuallyassociated with imaging.

The invention also relates to a method of detecting misalignment betweentwo layers of a device. The overlying or interlaced periodic structuresare illuminated by incident radiation. The diffracted radiation from theoverlying or interlaced periodic structures is used to provide an outputsignal. In one embodiment, a signal is derived from the output signal.The misalignment between the structures is determined from the outputsignal or the derived signal. In one embodiment, the output signal orthe derived signal is compared with a reference signal. A database thatcorrelates the misalignment with data related to diffracted radiationcan be constructed.

An advantage of this method is the use of only one incident radiationbeam. Another advantage of this method is the high sensitivity ofzero-order and first-order diffracted light to the overlaymisregistration between the layers. In particular, properties whichexhibited high sensitivity are intensity, phase and polarizationproperties of zero-order diffraction; differential intensity between thepositive and negative first-order diffraction; differential phasebetween the positive and negative first-order diffraction; anddifferential polarization between the positive and negative first-orderdiffraction. These properties also yielded linear graphs when plottedagainst the overlay misalignment. This method can be used to determinemisalignment on the order of nanometers.

In one embodiment, a neutral polarization angle, defined as an incidentpolarization angle where the differential intensity is equal to zero forall overlay misregistrations, is determined. The slope of differentialintensity as a function of incident polarization angle is highly linearwhen plotted against the overlay misregistration. This linear behaviorreduces the number of parameters that need to be determined anddecreases the polarization scanning needed. Thus, the method ofdetecting misalignment is faster when using the slope measurementtechnique.

The invention also relates to an apparatus for detecting misalignment ofoverlying or interlaced periodic structures. The apparatus comprises asource, at least one analyzer, at least one detector, and a signalprocessor to determine misalignment of overlying or interlaced periodicstructures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 h are cross-sectional views illustrating basic process stepsin semiconductor processing.

FIG. 2 a is a cross-sectional view of two overlying periodic structures.FIGS. 2 b and 2 c are top views of the two overlying periodic structuresof FIG. 2 a.

FIG. 3 is a top view of two overlying periodic structures illustratingan embodiment of the invention.

FIGS. 4 a and 4 b are cross-sectional views of overlying or interlacedperiodic structures illustrating other embodiments of the invention.

FIGS. 5 a and 5 b are cross-sectional views of two interlaced periodicstructures illustrating interlaced gratings in an embodiment of theinvention.

FIG. 6 is a cross-sectional view of two interlaced periodic structuresillustrating interlaced gratings in another embodiment of the invention.

FIGS. 7 a and 7 b are schematic views illustrating negative and positiveoverlay shift, respectively.

FIG. 8 is a schematic view illustrating the diffraction of light from agrating structure.

FIG. 9 a is a schematic block diagram of an optical system that measureszero-order diffraction from overlying or interlaced periodic structures.FIG. 9 b is a schematic block diagram of an integrated system of theoptical system of FIG. 9 a and a deposition instrument.

FIGS. 10 a and 11 a are schematic block diagrams of an optical systemthat measures first-order diffraction from a normal incident beam onoverlying or interlaced periodic structures. FIGS. 10 b and 11 b areschematic block diagrams of integrated systems of the optical systems ofFIGS. 10 a and 11 a, respectively, and a deposition instrument.

FIGS. 12 a and 12 b are graphical plots of derived signals fromzero-order diffraction of incident radiation on overlying structures.

FIGS. 13-14 and 16-17 are graphical plots of derived signals fromfirst-order diffraction of incident radiation on overlying structures.FIG. 15 is a graphical plot illustrating the mean square error.

FIGS. 18-19 and 21-22 are graphical plots of derived signals fromzero-order diffraction of incident radiation on interlaced gratings.FIGS. 20 and 23 are graphical plots illustrating the mean square error.

FIG. 24 is a graphical plot illustrating the determination ofmisalignment from a slope near a neutral polarization angle.

For simplicity of description, identical components are labeled by thesame numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 a is a cross-sectional view of a target 11 comprising twoperiodic structures 13, 15 on two layers 31, 33 of a device 17. Thesecond periodic structure 15 is overlying or interlaced with the firstperiodic structure 13. The layers and the periodic structures may be atthe same or different heights. The device 17 can be any device of whichthe alignment between two layers, particularly layers having smallfeatures on structures, needs to be determined. These devices aretypically semiconductor devices; thin films for magnetic heads for datastorage devices such as tape recorders; and flat panel displays.

As shown in FIGS. 1 a-1 h, a device 17 is generally formed in a basicseries of steps for each layer. First, as shown in FIG. 1 a, a layer 2is formed on a semiconductor substrate 1. The layer 2 may be formed byoxidization, diffusion, implantation, evaporation, or deposition.Second, as shown in FIG. 1 b, resist 3 is deposited on the layer 2.Third, as shown in FIG. 1 c, the resist 3 is selectively exposed to aform of radiation 5. This selective exposure is accomplished with anexposure tool and mask 4, or data tape in electron or ion beamlithography (not shown). Fourth, as shown in FIG. 1 d, the resist 3 isdeveloped. The resist 3 protects the regions 6 of the layer 2 that itcovers. Fifth, as shown in FIG. 1 e, the exposed regions 7 of the layer2 are etched away. Sixth, as shown in FIG. 1 f, the resist 3 is removed.Alternatively, in another embodiment, another material 8 can bedeposited in the spaces 7, as shown in FIG. 1 e, of the etched layer 2,as shown in FIG. 1 g, and the resist 3 is removed after the deposition,as shown in FIG. 1 h. This basic series of steps is repeated for eachlayer until the desired device is formed.

A first layer 31 and a second layer 33 can be any layer in the device.Unpatterned semiconductor, metal or dielectric layers may be depositedor grown on top of, underneath, or between the first layer 31 and thesecond layer 33.

The pattern for the first periodic structure 13 is in the same mask asthe pattern for a first layer 31 of the device, and the pattern for thesecond periodic structure 15 is in the same mask as the pattern for asecond layer 33 of the device. In one embodiment, the first periodicstructure 13 or the second periodic structure 15 is the etched spaces 7of the first layer 31 or the second layer 33, respectively, as shown inFIG. 1 f. In another embodiment the first periodic structure 13 or thesecond periodic structure 15 is the lines 2 of the first layer 31 or thesecond layer 33, respectively, as shown in FIG. 1 f. In anotherembodiment the first periodic structure 13 or the second periodicstructure 15 is another material 8 deposited in the spaces 7 of thefirst layer 31 or the second layer 33, respectively, as shown in FIG. 1h. In yet another embodiment, the second layer 33 is resist, and thesecond periodic structure 15 is resist 3 gratings, as shown in FIG. 1 d.

The first periodic structure 13 has the same alignment as the firstlayer 31, since the same mask was used for the pattern for the firstperiodic structure 13 and for the pattern for the first layer 31.Similarly, the second periodic structure 15 has the same alignment asthe second layer 33. Thus, any overlay misregistration error in thealignment between the first layer 31 and the second layer 33 will bereflected in the alignment between the first periodic structure 13 andthe second periodic structure 15.

FIGS. 2 b and 2 c are top views of target 11. In one embodiment, asillustrated in FIG. 2 a, the first periodic structure 13 has a firstselected width CD1, and the second periodic structure 15 has a secondselected width CD2. The second selected width CD2 is less than the firstselected width CD1. The pitch, also called the period or the unit cell,of a periodic structure is the distance after which the pattern isrepeated. The distance between the left edge of the first periodicstructure 13 and the left edge of the second periodic structure 15 isd₁, and the distance between the right edge of the first periodicstructure 13 and the right edge of the second periodic structure 15 isd₂. In a preferred embodiment, when layers 31, 33 are properly alignedrelative to each other, the second periodic structure 15 is centeredover the first periodic structure 13. In other words, when the secondperiodic structure 15 is perfectly centered over the first periodicstructure 13, the misregistration is zero, and d₁=d₂. In thisembodiment, the misregistration is indicated by d₂−d₁. To obtainmisregistration in both the X and Y directions of the XY coordinatesystem, another target 12 comprising two periodic structures 14, 16similar to target 11 is placed substantially perpendicular to target 11,as shown in FIG. 2 c.

The target 11 is particularly desirable for use in photolithography,where the first layer 31 is exposed to radiation for patterning purposesof a semiconductor wafer and the second layer 33 is resist. In oneembodiment, the first layer 31 is etched silicon, and the second layer33 is resist.

FIGS. 4 a and 4 b show alternative embodiments. In one embodiment, FIG.4 a illustrates a first periodic structure 13 of oxide having atrapezoidal shape on a first layer 31 of silicon substrate and a secondperiodic structure 15 of resist with a second layer 33 of resist. Thefirst layer 31 of silicon is etched, and shallow trench isolation(“STI”) oxide is deposited in the spaces of the etched silicon. Thelines of STI oxide form the first periodic structure 13. An oxide layer34 and a uniform polysilicon layer 35 are deposited between the firstlayer 31 of silicon and the second layer 33 of resist. The configurationin FIG. 4 a shows a line on space configuration, where the secondperiodic structure 15 is placed aligned with the spaces between thefirst periodic structure 13. The invention also encompasses embodimentssuch as the line on line configuration, where the lines in the secondperiodic structure 15 are placed on top of and aligned with the lines inthe first periodic structure 13, as shown by the dotted lines in FIG. 4a.

In another embodiment, FIG. 4 b illustrates a first periodic structure13 of tungsten etched in a first layer 31 of oxide and a second periodicstructure 15 of resist with a second layer 33 of resist. The first layer31 and the second layer 33 are separated by an aluminum blanket 37.

The invention relates to a method of making a target 11. A firstperiodic structure 13 is placed over a first layer 31 of a device 17. Asecond periodic structure 15 is placed over a second layer 33 of thedevice 17. The second periodic structure 15 is overlying or interlacedwith the first periodic structure 13.

In one embodiment, another target 12 is placed substantiallyperpendicular to target 11, as shown in FIG. 2 c. A third periodicstructure 14 is placed over the first layer 31, and a fourth periodicstructure 14 is placed over the second layer 33. The third periodicstructure 14 is substantially perpendicular to the first periodicstructure 13, and the fourth periodic structure 16 is substantiallyperpendicular to the second periodic structure 15.

An advantage of the target 11 is that the measurement of misregistrationof the target is free from optical asymmetries usually associated withimaging. Another advantage of this measurement is that it does notrequire scanning over the target as it is done with other techniques,such as in Bareket, U.S. Pat. No. 6,023,338. Another advantage of thetarget 11 is the elimination of a separate diffraction system and adifferent target to measure the critical dimension (“CD”) of a periodicstructure. The critical dimension, or a selected width of a periodicstructure, is one of many target parameters needed to calculatemisregistration. Using the same diffraction system and the same targetto measure both the overlay misregistration and the CD is moreefficient. The sensitivity associated with the CD and that with themisregistration is distinguished by using an embodiment of a target asshown in FIG. 3. The second periodic structure 15 extends further to anarea, the CD region 21, where the first periodic structure 13 does notextend. The first selected width CD1 is measured before placing thesecond periodic structure 15 on the device 17. After forming the target,the second selected width CD2 alone can be measured in the CD region 21.In a separate measurement, the misregistration is determined in anoverlay region 19 where both the first 13 and second 15 periodicstructures lie.

FIGS. 5 a and 5 b are cross-sectional views of an embodiment of a targethaving interlaced gratings. The first periodic structure 13 or thesecond periodic structure 15 has at least two interlaced grating lineshaving different periods, line widths or duty cycles. The first periodicstructure 13 is patterned with the same mask as that for the first layer31, and the second periodic structure 15 is patterned with the same maskas that for the second layer 33. Thus, the first periodic structure 13has the same alignment as the first layer 31, and the second periodicstructure 15 has the same alignment as the second layer 33. Anymisregistration between the first layer 31 and the second layer 33 isreflected in the misregistration between the first periodic structure 13and the second periodic structure 15.

In the embodiment shown in FIGS. 5 a and 5 b, the first periodicstructure 13 has two interlaced grating lines 51, 53. The firstinterlaced grating lines 51 have a line-width L₁, and the secondinterlaced grating lines 53 have a line-width L₂. The second periodicstructure 15, as shown in FIG. 5 b, has a line-width L₃ and is centeredbetween the first interlaced grating lines 51 and the second interlacedgrating lines 53. The distance between the right edge of the firstinterlaced grating 51 and the adjacent left edge of the secondinterlaced grating 53 is represented by b, and the distance between theright edge of the second periodic structure 15 and the adjacent leftedge of the second interlaced grating 53 is represented by c. Themisregistration between the first layer 31 and the second layer 33 isequal to the misregistration ∈ between the first periodic structure 13and the second periodic structure 15. The misregistration ∈ is:

$\begin{matrix}{ɛ = {\frac{b}{2} - \frac{L_{3}}{2} - c}} & (1)\end{matrix}$

Where c=0, the resulting periodic structure has the most asymmetric unitcell composed of a line with width of L₂+L₃ and a line with width L₁.Where c=b−L₃, the resulting periodic structure has the most symmetricunit cell composed of a line with width L₁+L₃ and a line with width L₂.For example, if the two layers are made of the same material andL₁=L₃=L₂/2, then the lines are identical where c=0, while one line istwice as wide as the other line where c=b−L₃.

FIG. 6 shows an alternative embodiment of a target having interlacedgratings. The first periodic structure 13 is etched silicon, and thesecond periodic target 15 is resist. The first layer 31 of siliconsubstrate and the second layer 33 of resist are separated by an oxidelayer 39.

The invention also relates to a method of making a target 11. A firstperiodic structure 13 is placed over a first layer 31 of a device 17. Asecond periodic structure 15 is placed over a second layer 33 of thedevice 17. The second periodic structure 15 is overlying or interlacedwith the first periodic structure 13. Either the first periodicstructure 13 or the second periodic structure 15 has at least twointerlaced grating lines having different periods, line widths or dutycycles.

An advantage of interlaced gratings is the ability to determine the signof the shift of the misregistration from the symmetry of the interlacedgratings. FIGS. 7 a and 7 b are schematic drawings illustrating negativeand positive overlay shift, respectively, in the X direction of the XYcoordinate system. Center line 61 is the center of a grating 63. Whenthe grating 63 is aligned perfectly, the center line 61 is aligned withthe Y axis of the XY coordinate system. As shown in FIG. 7 a, a negativeoverlay shift is indicated by the center line 61 being in the negative Xdirection. As shown in FIG. 7 b, a positive overlay shift is indicatedby the center line 61 being in the positive X direction. The negativeoverlay shift is indicated by a negative number for the misregistration,and the positive overlay shift is indicated by a positive number for themisregistration. The misregistration can be determined using the methoddiscussed below. In the case of the interlaced gratings, a negativeoverlay shift results in a more symmetrical unit cell, as where c=b−L₃,discussed above. A positive overlay shift results in a more asymmetricalunit cell, as where c=0, discussed above.

The invention relates to a method to determine misalignment usingdiffracted light. FIG. 8 is a schematic view showing the diffraction oflight from a grating structure 71. In one embodiment, incident radiation73 having an oblique angle of incidence θ illuminates the gratingstructure 71. The grating structure 71 diffracts radiation 75, 77, 79.Zero-order diffraction 75 is at the same oblique angle θ to thesubstrate as the incident radiation 73. Negative first-order diffraction77 and positive first-order diffraction 79 are also diffracted by thegrating structure 71.

Optical systems for determining misalignment of overlying or interlacedperiodic structures are illustrated in FIGS. 9 a, 10 a, and 11 a. FIG. 9a shows an optical system 100 using incident radiation beam 81 with anoblique angle of incidence and detecting zero-order diffracted radiation83. A source 102 provides polarized incident radiation beam 81 toilluminate periodic structures on a wafer 91. The incident radiationbeam may be substantially monochromatic or polychromatic. The source 102comprises a light source 101 and optionally acollimating/focusing/polarizing optical module 103. The structuresdiffract zero-order diffracted radiation 83. Acollimating/focusing/analyzing optical module 105 collects thezero-order diffracted radiation 83, and a light detection unit 107detects the zero-order diffracted radiation 83 collected by the analyzerin module 105 to provide an output signal 85. A signal processor 109determines any misalignment between the structures from the outputsignal 85. The output signal 85 is used directly to determinemisalignment from the intensity of the zero-order diffracted radiation83. In a preferred embodiment the misalignment is determined bycomparing the intensity with a reference signal, such as a referencesignal from a calibration wafer or a database, compiled as explainedbelow. In one embodiment, the signal processor 109 calculates a derivedsignal from the output signal 85 and determines misalignment from thederived signal. The derived signal can include polarization or phaseinformation. In this embodiment, the misalignment is determined bycomparing the derived signal with a reference signal.

In one embodiment, optical system 100 provides ellipsometric parametervalues, which are used to derive polarization and phase information. Inthis embodiment, the source 102 includes a light source 101 and apolarizer in module 103. Additionally, a device 104 causes relativerotational motion between the polarizer in module 103 and the analyzerin module 105. Device 104 is well known in the art and is not describedfor this reason. The polarization of the reflected light is measured bythe analyzer in module 105, and the signal processor 109 calculates theellipsometric parameter values, tan(Ψ) and cos(Δ), from the polarizationof the reflected light. The signal processor 109 uses the ellipsometricparameter values to derive polarization and phase information. The phaseis Δ. The polarization angle α is related to tan(Ψ) through thefollowing equation:

$\begin{matrix}{{\tan \; \alpha} = \frac{1}{\tan \; \Psi}} & (2)\end{matrix}$

The signal processor 109 determines misalignment from the polarizationor phase information, as discussed above.

The imaging and focusing of the optical system 100 in one embodiment isverified using the vision and pattern recognition system 115. The lightsource 101 provides a beam for imaging and focusing 87. The beam forimaging and focusing 87 is reflected by beam splitter 113 and focused bylens 111 to the wafer 91. The beam 87 then is reflected back through thelens 111 and beam splitter 113 to the vision and pattern recognitionsystem 115. The vision and pattern recognition system 115 then sends arecognition signal 88 for keeping the wafer in focus for measurement tothe signal processor 109.

FIG. 10 a illustrates an optical system 110 using normal incidentradiation beam 82 and detecting first-order diffracted radiation 93, 95.A source 202 provides polarized incident radiation beam 82 to illuminateperiodic structures on a wafer 91. In this embodiment, the source 202comprises a light source 101, a polarizer 117 and lens 111. Thestructures diffract positive first-order diffracted radiation 95 andnegative first-order diffracted radiation 93. Analyzers 121, 119 collectpositive first-order diffracted radiation 95 and negative first-orderdiffracted radiation 93, respectively. Light detection units 125, 123detect the positive first-order diffracted radiation 95 and the negativefirst-order diffracted radiation 93, respectively, collected byanalyzers 121, 119, respectively, to provide output signals 85. A signalprocessor 109 determines any misalignment between the structures fromthe output signals 85, preferably by comparing the output signals 85 toa reference signal. In one embodiment, the signal processor 109calculates a derived signal from the output signals 85. The derivedsignal is a differential signal between the positive first-orderdiffracted radiation 95 and the negative first-order diffractedradiation 93. The differential signal can indicate a differentialintensity, a differential polarization angle, or a differential phase.

Optical system 110 determines differential intensity, differentialpolarization angles, or differential phase. To determine differentialphase, optical system 110 in one embodiment uses an ellipsometricarrangement comprising a light source 101, a polarizer 117, an analyzer119 or 121, a light detector 123 or 125, and a device 104 that causesrelative rotational motion between the polarizer 117 and the analyzer119 or 121. Device 104 is well known in the art and is not described forthis reason. This arrangement provides ellipsometric parameters forpositive first-order diffracted radiation 95 and ellipsometricparameters for negative first-order diffracted radiation 93, which areused to derive phase for positive first-order diffracted radiation 95and phase for negative first-order diffracted radiation 93,respectively. As discussed above, one of the ellipsometric parameters iscos(Δ), and the phase is Δ. Differential phase is calculated bysubtracting the phase for the negative first-order diffracted radiation93 from the phase for the positive first-order diffracted radiation 95.

To determine differential polarization angles, in one embodiment, thepolarizer 117 is fixed for the incident radiation beam 82, and theanalyzers 121, 119 are rotated, or vice versa. The polarization anglefor the negative first-order diffracted radiation 93 is determined fromthe change in intensity as either the polarizer 117 or analyzer 119rotates. The polarization angle for the positive first-order diffractedradiation 95 is determined from the change in intensity as either thepolarizer 117 or analyzer 121 rotates. A differential polarization angleis calculated by subtracting the polarization angle for the negativefirst-order diffracted radiation 93 from the polarization angle for thepositive first-order diffracted radiation 95.

To determine differential intensity, in one embodiment, the analyzers119, 121 are positioned without relative rotation at the polarizationangle of the first-order diffracted radiation 93, 95. Preferably, at thepolarization angle where the intensity of the diffracted radiation is amaximum, the intensity of the positive first-order diffracted radiation95 and the intensity of the negative first-order diffracted intensity 93is detected by the detectors 125, 123. Differential intensity iscalculated by subtracting the intensity for the negative first-orderdiffracted radiation 93 from the intensity for the positive first-orderdiffracted radiation 95.

In another embodiment, the differential intensity is measured as afunction of the incident polarization angle. In this embodiment, thepolarizer 117 is rotated, and the analyzers 119, 121 are fixed. As thepolarizer 117 rotates, the incident polarization angle changes. Theintensity of the positive first-order diffracted radiation 95 and theintensity of the negative first-order diffracted radiation 93 isdetermined for different incident polarization angles. Differentialintensity is calculated by subtracting the intensity for the negativefirst-order diffracted radiation 93 from the intensity for the positivefirst-order diffracted radiation 95.

The imaging and focusing of the optical system 110 in one embodiment isverified using the vision and pattern recognition system 115. Afterincident radiation beam 82 illuminates the wafer 91, a light beam forimaging and focusing 87 is reflected through the lens 111, polarizer117, and beam splitter 113 to the vision and pattern recognition system115. The vision and pattern recognition system 115 then sends arecognition signal 88 for keeping the wafer in focus for measurement tothe signal processor 109.

FIG. 11 a illustrates an optical system 120 where first-order diffractedradiation beams 93, 95 are allowed to interfere. The light source 101,device 104, polarizer 117, lens 111, and analyzers 119, 121 operate thesame way in optical system 120 as they do in optical system 110. Device104 is well known in the art and is not described for this reason. Oncethe negative first-order diffracted radiation 93 and positivefirst-order diffracted radiation 95 are passed through the analyzers119, 112, respectively, a first device causes the positive first-orderdiffracted radiation 95 and the negative first-order diffractedradiation 93 to interfere. In this embodiment, the first devicecomprises a multi-aperture shutter 131 and a flat beam splitter 135. Themulti-aperture shutter 131 allows both the negative first-orderdiffracted radiation 93 and the positive first-order diffracted beam 95to pass through it. The flat beam splitter 135 combines the negativefirst-order diffracted radiation 93 and the positive first-orderdiffracted radiation 95. In this embodiment, the mirrors 127, 133 changethe direction of the positive first-order diffracted radiation 95. Alight detection unit 107 detects the interference 89 of the twodiffracted radiation signals to provide output signals 85. A signalprocessor 109 determines any misalignment between the structures fromthe output signals 85, preferably by comparing the output signals 85 toa reference signal. The output signals 85 contain information related tophase difference.

In one embodiment, phase shift interferometry is used to determinemisalignment. The phase modulator 129 shifts the phase of positivefirst-order diffracted radiation 95. This phase shift of the positivefirst-order diffracted radiation 95 allows the signal processor 109 touse a simple algorithm to calculate the phase difference between thephase for the positive first-order diffracted radiation 95 and the phasefor the negative first-order diffracted radiation 93.

Differential intensity and differential polarization angle can also bedetermined using optical system 120. The multi-aperture shutter 131operates in three modes. The first mode allows both the positivefirst-order diffracted radiation 95 and the negative first-orderdiffracted radiation 93 to pass through. In this mode, differentialphase is determined, as discussed above. The second mode allows only thepositive first-order diffracted radiation 95 to pass through. In thismode, the intensity and polarization angle for the positive first-orderdiffracted radiation 95 can be determined, as discussed above. The thirdmode allows only the negative first-order diffracted radiation 93 topass through. In this mode, the intensity and polarization angle for thenegative first-order diffracted radiation 93 can be determined, asdiscussed above.

To determine differential intensity, the multi-aperture shutter 131 isoperated in the second mode to determine intensity for positivefirst-order diffracted radiation 95 and then in the third mode todetermine intensity for negative first-order diffracted radiation 93, orvice versa. The differential intensity is then calculated by subtractingthe intensity of the negative first-order diffracted radiation 93 fromthe intensity of the positive first-order diffracted radiation 95. Thesignal processor 109 determines misalignment from the differentialintensity.

In one embodiment, the differential intensity is measured at differentincident polarization angles. The measurements result in a large set ofdata points, which, when compared to a reference signal, provide a highaccuracy in the determined value of the misregistration.

To determine differential polarization angle, the multi-aperture shutter131 is operated in the second mode to determine polarization angle forpositive first-order diffracted radiation 95 and then in the third modeto determine polarization angle for negative first-order diffractedradiation 93, or vice versa. The differential polarization angle is thencalculated by subtracting the polarization angle of the negativefirst-order diffracted radiation 93 from the polarization angle of thepositive first-order diffracted radiation 95. The signal processor 109determines misalignment from the differential polarization angle.

The imaging and focusing of the optical system 120 is verified using thevision and pattern recognition system 115 in the same way as the imagingand focusing of the optical system 110 is in FIG. 10. In one embodiment,the beam splitter 113 splits off radiation 89 to reference lightdetection unit 137, which detects fluctuations of the light source 101.The reference light detection unit 137 communicates information 86concerning intensity fluctuation of source 101 to the signal processingand computing unit 109. The signal processor 109 normalizes the outputsignal 85 using fluctuation information 86.

Optical systems 100, 110, 120 can be integrated with a depositioninstrument 200 to provide an integrated tool, as shown in FIGS. 9 b, 10b and 11 b. The deposition instrument 200 provides the overlying orinterlaced periodic structures on wafer 91 in step 301. Optical systems100, 110, 120 obtains misalignment information from the wafer 91 in step302. The signal processor 109 of optical systems 100, 110, 120 providesthe misalignment to the deposition tool 200 in step 303. The depositiontool uses the misalignment information to correct for any misalignmentbefore providing another layer or periodic structure on wafer 91 in step301.

Optical systems 100, 110, 120 are used to determine the misalignment ofoverlying or interlaced periodic structures. The source providingpolarized incident radiation beam illuminates the first periodicstructure 13 and the second periodic structure 15. Diffracted radiationfrom the illuminated portions of the overlying or interlaced periodicstructures are detected to provide an output signal 85. The misalignmentbetween the structures is determined from the output signal 85. In apreferred embodiment, the misalignment is determined by comparing theoutput signal 85 with a reference signal, such as a reference signalfrom a calibration wafer or a database, compiled as explained below.

The invention relates to a method for providing a database to determinemisalignment of overlying or interlaced periodic structures. Themisalignment of overlying or interlaced periodic structures andstructure parameters, such as thickness, refractive index, extinctioncoefficient, or critical dimension, are provided to calculate datarelated to radiation diffracted by the structures in response to a beamof radiation. The data can include intensity, polarization angle, orphase information. Calculations can be performed using known equationsor by a software package, such as Lambda SW, available from Lambda,University of Arizona, Tuscon, Ariz., or Gsolver SW, available fromGrating Solver Development Company, P.O. Box 353, Allen, Tex. 75013.Lambda SW uses eigenfunctions approach, described in P. Sheng, R. S.Stepleman, and P. N. Sandra, Exact Eigenfunctions for Square WaveGratings: Applications to Diffraction and Surface Plasmon Calculations,Phys. Rev. B, 2907-2916 (1982), or the modal approach, described in L.Li, A Modal Analysis of Lamellar Diffraction Gratings in ConicalMountings, J. Mod. Opt. 40, 553-573 (1993). Gsolver SW uses rigorouscoupled wave analysis, described in M. G. Moharam and T. K. Gaylord,Rigorous Coupled-Wave Analysis of Planar-Grating Diffraction, J. Opt.Soc. Am. 73, 1105-1112 (1983). The data is used to construct a databasecorrelating the misalignment and the data. The overlay misregistrationof a target can then be determined by comparing the output signal 85with the database.

FIGS. 12-24 were generated through computer simulations using either theLambda SW or the Gsolver SW. FIGS. 12 a and 12 b are graphical plotsillustrating the ellipsometric parameters obtained using an overlyingtarget of FIG. 2 a with the optical system of FIG. 9 a. The calculationswere performed using the Lambda SW. The overlying target used in themeasurement comprises first periodic structure 13 and the secondperiodic structure 15 made of resist gratings having 1 μm depth on asilicon substrate. The depth of the first periodic structure 13 and thesecond periodic structure 15 is 0.5 μm, and the pitch is 0.8 μm. Thefirst selected width CD1 for the first periodic structure 13 is 0.4 μm,and the second selected width CD2 for the second periodic structure 15is 0.2 μm. The incident beam in this embodiment was TE polarized. Thesetarget parameters and the overlay misregistration were inputted into theLambda SW to obtain ellipsometeric parameter values. The ellipsometricparameter values were obtained for zero-order diffracted radiation usingan incident radiation beam 81 at an angle of 25° to the wafer surface.The ellipsometric parameters, Tan [Ψ] and Cos [Δ], were plotted as afunction of the wavelengths in the spectral range 230 to 400 nanometers.The ellipsometric parameters are defined as:

$\begin{matrix}{{\tan \; \Psi} = \frac{r_{p}}{r_{s}}} & (3)\end{matrix}$

where r_(p) and r_(s) are the amplitude reflection coefficients for thep(TM) and s(TE) polarizations, and

Δ=φ_(p)−φ_(s)  (4)

where φ_(p) and φ_(s) are the phases for the p(TM) and s(TE)polarizations. Results were obtained for different values of overlaymisregistration d₂−d₁ varying from −15 nanometers to 15 nanometers insteps of 5 nanometers. The variations for tan [Ψ] and cos [Δ] showsensitivity to the misregistration in the nanometer scale. To get moreaccurate results, first-order diffracted radiation is detected usingnormal incident radiation, as in FIGS. 13-14.

FIGS. 13 and 14 are graphical plots illustrating the differentialintensity obtained using overlying targets of FIG. 2 a and an opticalsystem detecting first-order diffracted radiation using normal incidentradiation. The calculations were performed using Gsolver SW. The firstperiodic layer 13 is etched silicon, while the second periodic layer 15is resist. The overlay misregistration and target parameters wereinputted into Gsolver SW to obtain the differential intensity in FIGS.13 and 14. FIG. 13 shows the normalized differential intensity betweenthe positive and negative first-order diffracted radiation as a functionof overlay misregistrations. The differential intensity is defined as:

$\begin{matrix}{{DS} = {\frac{R_{+ 1} - R_{- 1}}{R_{+ 1} + R_{- 1}}\%}} & (5)\end{matrix}$

where R₊₁ is the intensity of the positive first-order diffractedradiation and R⁻¹ is the intensity of the negative first-orderdiffracted radiation. The different curves in FIG. 13 correspond to thedifferent incident polarization angles (0°, 50°, 60°, 74°, 80°, and 90°)of the incident linearly polarized light relative to the plane ofincidence. The polarization angle α is defined as:

$\begin{matrix}{\alpha = {\arctan \left( \frac{E_{s}}{E_{p}} \right)}} & (6)\end{matrix}$

where E_(s) is the field component perpendicular to the plane ofincidence, which for normal incidence is the Y component in the XYcoordinate system, and E_(p) is the field component parallel to theplane of incidence, which for normal incidence is the X component.Polarization scans from incident polarization angles of 0° to 90° wereperformed to generate the graphical plots in FIGS. 13 and 14. FIG. 14shows the differential intensity as a function of incident polarizationangle at different overlay misregistration (−50 nm, −35 nm, −15 nm, 0nm, 15 nm, 35 nm, and 50 nm). FIG. 14 shows that there is a neutralpolarization angle, defined as an incident polarization angle where thedifferential intensity is equal to zero for all overlay misregistration.FIGS. 13 and 14 illustrate the high sensitivity of differentialintensity to the overlay misregistration and the linear behavior ofdifferential intensity with the overlay misregistration. They also showthat the differential intensity is zero at zero overlay misregistrationfor any polarization angle. Similar graphical plots were obtained atdifferent wavelengths, FIG. 15 shows the mean square error (“MSE”)variation with the overlay misregistration. The MSE exhibits linearityand sensitivity of approximately 0.6 per one nanometer overlaymisregistration.

FIGS. 16 and 17 are graphical plots, using the same target withdifferent structure parameters and the same optical system as the onesin FIGS. 13 and 14. However, the calculations were performed using theLambda SW, instead of the Gsolver SW. The kinks or the deviations fromthe montonicity of the curves at certain points in FIGS. 16 and 17 arebelieved to be due to numerical instabilities frequently known to occurin the use of the Lambda SW. The overlay misregistration and the targetparameters were inputted into Lambda SW to obtain differentialpolarization angle and differential phase in FIGS. 16 and 17,respectively. FIG. 16 shows the variation of the difference between thepolarization angles of the positive and negative first-order diffractedradiation as a function of overlay misregistration for differentincident polarization angles (0°, 5°, 15°, 30°, 45°, 60°, and 90°). FIG.17 shows the variation of the difference between the phase angles of thepositive and negative first-order diffracted radiation. The phase anglehere represents the phase difference between the p and s polarizedcomponents of the diffracted light.

FIGS. 16 and 17 also illustrate the high sensitivity of differentialpolarization angle and differential phase, respectively, to the overlaymisregistration and the linear behavior of differential polarizationangle and differential phase, respectively, when plotted against theoverlay misregistration. They also show that the differentialpolarization angle and differential phase is zero at zero overlaymisregistration for any polarization angle. However, FIG. 17 shows thatthe phase difference does not depend on incident polarization. In oneembodiment, the difference between the polarization angles, as shown inFIG. 16, is easily measured with an analyzer at the output, while thephase difference, as shown in FIG. 17, is measured with interferometry.In another embodiment, the differential polarization angle and thedifferential phase is derived from ellipsometric parameters.

Similar results were obtained using the overlying targets in FIGS. 4 aand 4 b. However, for the particular target in FIG. 4 a, there was noneutral polarization angle in the line on line configuration, where thesecond periodic structure 15 is centered on the first periodic structure13. The line on space configuration, where the second periodic structure15 is centered on the spaces between the first periodic structure 13,did exhibit a neutral polarization angle. These results show that theneutral polarization angle apparently has a complicated dependence onthe structure parameters.

FIGS. 18-19 and 21-22 are graphical plots illustrating the intensity ofthe zero-order diffracted radiation 83, as shown in FIG. 9 a, forinterlaced gratings, as shown in FIG. 6. Table 1 summarizes theparameters used in the calculations by the Gsolver SW.

TABLE 1 Structure parameters used in the simulations Parameter Data76Data0 h1 850 nm 850 nm h2 850 nm 850 nm h3 600 nm 600 nm Pitch (P) 1000nm  2000 nm  CD1 150 nm 200 nm CD2 300 nm 600 nm CD3 150 nm 200 nmIncidence angle (θ) 76° 0 Azimuth angle (φ) 0 0 Wavelength (λ) 670 nm500 nmThe incidence angle is 76° in the Data76 configuration, and theincidence angle is 0° (normal) in the Data0 configuration.

FIGS. 18-20 were derived using the Data76 configuration. FIG. 18 showsthe intensity of the zero-order diffracted radiation versus the overlaymisregistration at different polarization angles (0° to 90° in steps of15°). Within a range of 140 nm, the changes are monotonic with theoverlay misregistration. The point where all the curves cross is at anoverlay misregistration value of 50 nm, rather than zero. At an overlaymisregistration value of 50 nm, the structure is effectively mostsymmetric. In contrast, in an overlying target as in FIG. 2 a, thestructure is most symmetric at zero overlay misregistration. FIG. 19shows the dependence of the intensity of the zero-order diffractedradiation on the incident polarization angle at different overlaymisregistrations (−50 nm, −15 nm, 0 nm, 20 nm, 40 nm, 60 nm, 80 nm, 100nm, and 130 nm). Unlike with the differential intensity of thefirst-order diffracted radiation, there is not a neutral polarizationangle where the differential intensity is zero for different overlaymisregistration. However, there is a quasi-neutral polarization anglewhere most of the curves for different misregistration cross. FIG. 20shows the MSE variation as a function of overlay misregistration. FIGS.18 and 19 show the high sensitivity of the intensity of zero-orderdiffracted radiation to the overlay sign for a configuration usingincident radiation having an oblique angle of incidence on interlacedgratings. They also show the linear behavior of the intensity whenplotted against the overlay misregistration.

FIGS. 21-23 were derived using the Data0 configuration. FIG. 21 showsthe intensity of the zero-order diffracted radiation versus the overlaymisregistration at different polarization angles (0°, 40°, 65°, and90°). FIG. 22 shows the dependence of the intensity of the zero-orderdiffracted radiation on the incident polarization angle at differentoverlay misregistrations (−140 nm, −100 nm, −50 nm, 0 nm, 50 nm, and 100nm). FIG. 23 shows the MSE variation as a function of overlaymisregistration. FIGS. 21 and 22 show the high sensitivity of theintensity of zero-order diffracted radiation to the overlay sign for aconfiguration using normal incident radiation on interlaced gratings.They also show the linear behavior of the intensity when plotted againstthe overlay misregistration.

FIG. 24 is a graphical plot generated by the Gsolver SW illustrating thedetermination of misalignment from the neutral polarization angle. Asshown in FIG. 14, the differential intensity equals zero independent ofthe overlay misregistration at the neutral polarization angle. However,the slope of the differential intensity varies with overlaymisregistration. FIG. 24 shows the slope near the neutral polarizationangle as a function of overlay misregistration. FIG. 24 shows linearbehavior of the slope versus the overlay misregistration with a slope of0.038% per 1 nm overlay misregistration. An advantage of the slopemeasurement technique is the reduction of the number of parameters thatneed to be determined. Another advantage is the decreased polarizationscanning needed. In FIG. 14, a polarization scan using incidentpolarization angles from 0° to 90° is performed. In contrast, using theslope measurement technique in one embodiment the derived signal iscompared with the reference signal for polarization angles within aboutfive degrees of the neutral polarization angle. Thus, the method ofdetecting misalignment is faster when using the slope measurementtechnique. Another embodiment of the invention is the use of the slopemeasurement technique for the quasi-neutral polarization angle.

Misalignment of overlying or interlaced periodic structures can bedetermined using the database in a preferred embodiment. The sourceproviding polarized incident radiation illuminates the first periodicstructure 13 and the second periodic structure 15. Diffracted radiationfrom the illuminated portions of the overlying or interlaced periodicstructures are detected to provide an output signal 85. The outputsignal 85 is compared with the database to determine the misalignmentbetween the overlying or interlaced periodic structures.

In another embodiment, misalignment of overlying or interlaced periodicstructures is determined using the slope measurement technique. Aneutral polarization angle or quasi-neutral polarization angle isprovided. The derived signal is compared with the reference signal nearthe neutral polarization angle or the quasi-neutral polarization angleto determine misalignment of the overlying or interlaced periodicstructures.

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalent. All referencesreferred to herein are incorporated by reference.

1. A target for measuring the relative positions between two layers of adevice, said target comprising: a first periodic structure over a firstlayer of the device; and a second periodic structure over a second layerof the device, said second periodic structure overlying or interlacedwith said first periodic structure, wherein said second periodicstructure includes a first and a second portion, wherein the firstportion is in a region overlying the first periodic structure and thesecond portion is in a region not overlying the first periodicstructure.
 2. The target of claim 1, wherein said second periodicstructure extends into an area where said first periodic structure doesnot extend.
 3. A target for measuring the relative positions between twolayers of a device, said target comprising: a first periodic structureover a first layer of the device; and a second periodic structure over asecond layer of the device, said second periodic structure overlyingwith said first periodic structure; wherein said first or periodicstructure has at least two interlaced grating lines having differentperiods, line widths or duty cycles.
 4. The target of claim 3, said atleast two interlaced grating lines having different duty cycles.
 5. Thetarget of claim 3, said at least two interlaced grating lines havingdifferent line widths.
 6. A method for detecting misalignment between afirst periodic structure and a second periodic structure of a device,said second periodic structure overlying or interlaced with said firstperiodic structure, comprising: measuring critical dimensions of thefirst and second periodic structures, wherein measurement of at leastone of the first and second periodic structures is performed withoutmeasuring the other one of the first and second periodic structures; andmeasuring the misalignment between the first and second periodicstructures.
 7. The method of claim 6, wherein said second periodicstructure includes a first and a second portion, wherein the firstportion is in a region overlying the first periodic structure and thesecond portion is in a region not overlying the first periodicstructure, and said critical dimension of the second periodic structureis measured by measuring said second portion of said second periodicstructure.